The solution film-forming method can produce superior films to those produced by the melt extrusion method with respect to optical properties as well as uniformity of the film thickness. Also because fine filtering is available, the film produced by solution film-forming method contains less extraneous materials. For these reasons, the solution film-forming method is widely adopted for manufacturing many kinds of films that are applied to optical devices, such as films for liquid crystal displays (LCD), films for organic electroluminescence (EL) displays, films for polarizing plates and films for photosensitive materials.
According to this method, a polymer material is dissolved with a solvent to produce a dope, and the dope is cast from a die onto a support to form a cast web, wherein the support is a metallic band or drum. The cast web is peeled off the support afterwards, and conveyed to a tenter drying process. In the tenter drying process, opposite lateral sides of the cast web are held by holding members, such as clips, to dry the web while stretching it. In this way, a film is produced.
However, there has been a problem that the cast web can wrinkle or curl as it is peeled off the support. If the cast web curls, it can be folded or interfere with the holders as it is held at its lateral sides by the holding members. This will result in unstable holding conditions. If the cast web wrinkles, the cast web will get narrower, so the holding condition of the cast web in the tenter drying process becomes unstable. The unstable condition can cause interruption of the manufacturing process or damage the film quality.
The higher the content of the solvent in the cast web and thus the lower the rigidity of the cast web at the time of peeling, such a trouble is more likely to occur. Also where there is a large difference in the solvent content between opposite surfaces of the cast web, there is a large difference in shrinking power between the opposite surfaces, so the wrinkles and the curls get remarkable. It is also known in the art that the wider the cast web, the more the cast web is likely to get wrinkled or curled.
To solve this problem, it may be possible to set the width of the cast web narrower, or use such a low casting speed that the cast web may be peeled off after the solvent sufficiently vaporizes from the web. But these methods will lower the productivity. It may also be possible to change the components of the dope to solve the above problem. But this involves a risk of damaging optical properties of the film products in exchange for reduction of wrinkles and curls.
To solve the above problem, Japanese Laid-open Patent Application No. 2003-33933 disposes a solution film-forming method, wherein guide plates are provided at the entrance of the tenter drying process, for correcting curls on opposite lateral sides of the cast web so that the web or film can be held stably through the tenter drying process.
Japanese Laid-open Patent Application No. 11-90942 discloses a solution film-forming method wherein opposite lateral sides of the cast web are cut away after the web is peeled off, and before the web is conveyed to the tenter drying process.
Although the prior method can prevent curling at the lateral sides of the cast web, this method cannot correct the wrinkles formed in a laterally intermediate portion of the cast web.
The method of the above Japanese Laid-open Patent Application No. 11-90942 will lower the productivity because it needs cutting of the lateral sides of the cast web.